If the twist rate of your barrel is on the order of 1 turn in 60 inchs or greater you've got a round ball barrel, though some rifles having a twist as fast as 1 turn in 48 inches will shoot round balls surprisingly well.
There are a couple combinations of ball/patch worth trying.
1. 0.433/0.435" ball with 0.015" patch material.
2. 0.440" ball with 0.012" patch material. This patch material is sometimes readily available, sometimes harder to find than 0.015".
If both of these combinations prove too hard to load try a 0.440" ball with 0.010" patch material.
In working up your load the rifle will reveal what it will shoot well.
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You'll almost certainly find that you get the easiest, fastest, and most consistent ignition using 4fg powder in the pan, though 3fg will work in a pinch. Don't fill the pan, fill the flash pan only about 1/3 to at most 2/3 "full". Too much powder will actually slow ignition, and reduce reliable ignition of the main charge.
Likewise, black powder used in the main charge is far easier to ignite than are any of the substitutes, and I believe that I've tried all of the currently available "black powder replacement" products. Personally I prefer to use 2f for the main charge in 45 calibre rifles if I can secure a "moist" burning powder, but I have used slightly smaller charges of 3f with success.
IMO, good English flints are very hard to beat. Also, a well polished (use crocus cloth or very fine abrasive in rubberized sticks (Cartex and Norton make these)) hammer (frizzen) will provide a more reliable spark and the flint will last much longer. Don't abrade/sand the hammer (frizzen), polish it, you want to put a shine on the steel, not remove material; this part is case hardened, often the case isn't very thick.
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Before you commence loading be sure that the ignition components, touchhole and barrel are absolutely free of oil, then insert a pick into the touchhole. Commercial prodicts are available, but the quill of a feather of appropriate size works well. To load, start with a modest charge, say 50 grains by volume. Transfer the powder from your flask to a charging measure after making certain that there are no bits of smoldering material within the rifle. Pour the powder down the barrel using a small funnel if you wish. If you're going to use a cast ball remember that you want to load "sprue up". Swaged balls don't show a sprue, orientation doesn't matter with them. If you're using precut patches be sure to center both the patch and the ball carefully, if your using a "hank" of material use a corner leaving sufficient to cut. Start the ball with your thumb, then use the short "tit" on a 45 calibre ball starter to start the ball into the barrel by bumping the knob of the starter with the ball of your hand once. This is the time to trim your patch if you're not using precuts. Don't use a 50 calibre starter, or some other size, you'll deform the ball. With the loading rod grapsed in both hands close to the muzzle, ease the ball down maintaining steady pressure until it seats firmly on the powder. Keep the muzzle pointed in a safe direction, always. Be sure that it is fully seated (marking the rod after load development helps assure this), but do not bounce the rod on the ball, you'll just deform the lead by doing this. At this point remove the faether or pick from the touchhole gently. Move the cock to the half cock position, move the hammer (frizzen) to the forward/open position and prime the pan. Personally I prefer to use a "pan charger" for this operation but you'll find opinion split among rifleman on the value/drawbacks of this accessory. Close the hammer (frizzen) and move the cock to the full cock position.
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Unless your rifle has pan and touchhole features used at the close of the flint era by the very best makers in England and France (precious metal inlays are a giveaway, but the shape of the components changed somewhat too) its best to keep the powder in the pan toward the outside edge, again, this will aide in reliable ignition.
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Many good .45 calibre (roundball) rifles shoot roundballs quite well with a charge of about 70 grains of black powder (by volume). By quite well, I mean five shot groups (shot from a bench) of one ragged hole at 50 yards, if cleaned as necessary. Work up in 5 or 10 grain increments, but never exceed the maximum charge recommended by the maker.
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To clean the rifle I simply use hot water, being sure to thoroughly dry then oil the bore before the barrel cools. Do use a rag to protect your hand from both the scalding water and the heat the barrel will absorb throught cleaning. Regardless of what the advertising folks say, a thorough cleanup of a rifle that has been used with black powder takes no longer than the same rifle used with any of the substitutes I've used, about 10 minutes once the water is hot enough (a bit longer on a double, and about 25 minutes for a cap and ball revolver- the powder fouling there is like sand on the beach- it gets everywhere).
best,
Bob